A method and apparatus for transmitting a power headroom (PH) report by a terminal in a mobile communication system supporting carrier aggregation are provided. The method includes determining respective PHs for multiple activated serving cells based on a real transmission or a reference format, setting respective indicators for the PHs, wherein each indicator indicates that a corresponding PH is determined based on the real transmission or the reference format, and transmitting, to a base station, an extended PH report including the PHs and the indicators on one of the multiple activated serving cells. The each indicator is set to “0”, if the corresponding PH is determined based on the real transmission, and the each indicator is set to “1”, if the corresponding PH is determined based on the reference format.